1. Field of the Invention
The present invention relates to an optical circuit and, more particularly, to an optical circuit including signal light level monitors and a monitor characteristic measurement method.
2. Description of the Related Art
An example of an optical circuit including optical amplifiers is shown in FIG. 1, which is based on a conventional optical amplification device disclosed in, for example, Japanese Patent Application Unexamined Publication No. Hei10-223963 (hereinafter, referred to as JP10-223963) and Japanese Patent Application Unexamined Publication No. 2002-374023A (hereinafter, referred to as JP2002-374023). This optical circuit can be broadly divided into two sections: an optical module section 103 that amplifies an optical signal; and an electronic control section 104 that controls the optical module section 103.
The optical module section 103 includes: an optical connector 51 for input of signal light into the optical module section 103; an optical connector 52 for output of the signal light from the optical module section 103; erbium-doped fibers (EDFs) 53 and 54, which are media for directly optically amplifying the signal light; a gain equalizer 55 for equalizing the signal light power of each channel included in the wavelength-division-multiplexed (WDM) signal light to be output from the optical module section 103; photo diodes (PDs) 56 to 59 for detecting the power of the signal light before and after each of the EDFs 53 and 54; and optical couplers 60 to 63 for branching the signal light into the PDs 56 to 59, respectively.
The electronic control section 104 includes: conversion circuits 64 to 67 for converting the currents output from the PDs 56 to 59 to output numerical data (PD monitored values), respectively; and a processor 68 for calculating the power of the monitored signal light from the PD monitored value (numerical data) inputted from each of the conversion circuits 64 to 67. Note that the processor 68 has a function of controlling the amplification factor for the signal light by adjusting, based on the PD monitored values, pumping light output from a pumping laser diode (LD) (not shown). Since this function is well known to those skilled in the art, a detailed description thereof will be omitted.
It is well known to those skilled in the art that the amount of photocurrent [μA] output from a PD in an optical module to be mounted on such an optical amplification device is measured in a process of manufacturing the optical module, to measure the amount of photocurrent [μA] for 1 mW of signal light power, conversion efficiency [μA/mW], whereby the correlation between the signal light power and the amount of PD output current is calculated.
Moreover, applying this scheme, it is also possible to measure the correlation between the signal light power and its PD monitored value that is numerical data obtained by a conversion circuit (for example, an AD converter or the like) converting the amount of photocurrent. Based on this correlation, a processor (for example, a CPU or the like) can calculate the power of monitored signal light from a corresponding PD monitored value.
According to the circuit structure as shown in FIG. 1, with respect to the PD 56 at its input stage and the PD 59 at its output stage, the signal light power can be directly measured with measurement equipment (not shown) inputting the signal light from the optical connectors 51 and 52, respectively, whereby it is possible to determine a relational expression between the power of each monitored signal light and the PD monitored value output from a corresponding one of the conversion circuits 64 and 67.
However, according to the circuit structure as shown in FIG. 1, for the PDs 57 and 58, a relational expression between monitored signal light power and its PD monitored value cannot be determined because the monitored signal light power cannot be directly measured. If one wishes to directly measure the monitored signal light power, measurement would be performed by cutting the optical waveguide at the input end of the PD (the optical fiber immediately before the PD) and connecting an optical power meter or the like thereto.
According to JP2002-374023, for a combination of the PD (57, 58) and the conversion circuit (65, 66), a number of designated light levels are imaginarily set for calculating PD characteristic values by using a characteristic equation between a level of input light to the PD and an output value of the conversion section. The processor 68 calculates a proportional. constant from the designated light levels and the corresponding PD characteristic values to obtain a linear function. However, such a method for obtaining the linear function puts a heavy processing load on the processor 68.